1. Field of the Invention
The present invention relates to a thin film electroluminescent (hereinafter abbreviated to "EL") element which is used in display devices, etc.
2. Description of the Related Art
It is indispensable to employ a display device as an interface between men and machines. For instance, in display devices of OA (i.e., office automation) equipment adapted for office work, computation, etc., a CRT (i.e., cathode ray tube) has been widely used because it allows high resolving power and good visibility, and because it displays full color with ease. However, the CRT suffers from the following drawback: since it is necessary to provide the CRT with a large depth, the CRT inevitably comes to occupy a large space.
On the other hand, there have been increasing demands for lap-top type personal computers. Accordingly, display devices are being reduced in size and weight, and even full-colored liquid crystals have come to be used. However, in the case of liquid crystal display devices, there are drawbacks as follows: since the liquid crystals themselves are non-luminous, it is necessary to provide the liquid crystal display devices with back lighting. Since liquid crystal display devices utilize the polarization of the liquid crystals, their visual field becomes smaller and they respond to commands slowly.
As display devices other than the CRT, the following display devices are used: plasma display devices, fluorescent display tubes, display tubes in which light-emitting diodes, are disposed in an array. However, these display devices have drawbacks in that they cannot be made to display full color because it is hard to produce all of the three primary colors of light, e.g., red, green and blue, with them. Further, the display dots of these display devices cannot be made smaller in view of their resolving power.
Under the aforementioned circumstances, EL elements has come to draw electronics engineers' attention: an EL element is a whole solid thin film which is self-luminous. The EL element has been put into practical application as a flat panel display device. This thin film EL element is of a whole solid type, and it has a construction in which a luminous layer is held between insulator layers.
When the EL element is combined with a transparent electrode, the EL element can be formed on an inexpensive glass substrate, and it enables surface luminescence over a large area. Further, when both of the electrodes on the top and bottom of the luminous layer are transparent electrodes, the EL element can be converted into a transparent type element. Furthermore, when photolithography is used to form the EL element, an extremely tiny EL element can be formed, and, accordingly, its resolving power can be increased.
In particular, a transparent element including a pair of transparent electrodes can be employed as an automobile indicator. If such is the case, the EL element enables the construction of a head up display device. The head up display device allows a driver to recognize the view field in front of him when the transparent element does not emit light, and it also allows him to recognize both the view field as well as the displayed information at the same time when the transparent element emits lights. Hence, the development of the head up display device is now under way.
Moreover, it is necessary to apply an alternating-current (hereinafter abbreviated to "AC") voltage of approximately 100 to 200 V in order to operate the EL element. Here, in the EL element, the luminous layer is held between insulator layers like a sandwich, and accordingly, electric current is regulated by them. As a result, the EL element consumes an extremely small amount of electric power.
In addition, the thin film EL element is self-luminous unlike conventional elements, and accordingly it is also of superior visibility. Hence, the thin film EL element is more appropriate for automobile display devices than conventional elements are.
However, there are the following problems with thin film EL elements at present. Namely, an AC electric field of an extremely high intensity should be applied to the thin film EL element according to the operational principle of the element. Therefore, in the construction of the element, the insulator layers hold the luminous layer like a sandwich, and they regulate the flow of electric current. Accordingly, the dielectric breakdown of the insulator layers is one factor which determines the life of the element. In addition, when the element is mounted on an automobile, it is used in a severe environment, for instance, in a high temperature and high humidity environment, which differs remarkably from current applications in general offices. In such an environment the insulator layers degrade rapidly. For an element having a large area, even when one of the pixels breaks down, the element hardly performs its functions properly. Hence, a system which enables the formation of insulator layers having high insulation properties over large areas uniformly becomes important. Also, the film forming technique used for forming the insulator layers is one of the key factors in improving the life of the element and the reliability thereof as well as in improving the yield loss in the production process of the element.
Generally speaking, a thin film EL element comprises a luminous layer including a semiconductor, which contains a compound including at least a combination of a group II element and a group VI element of the Periodic Table, such as ZnS, CaS, SrS, or the like as a base material to which a rare earth element is added, insulator layers holding the luminous layer like a sandwich therebetween, and transparent electrodes or metallic electrodes disposed on both surfaces of the insulator layers. When an AC voltage is applied between the electrodes, luminous centers of the luminous layers are excited, and consequently the electroluminescense is carried out.
A high electric field of approximately 1 to 2 MV/cm is applied to the luminous layer. Accordingly, the electric current must be regulated by the insulator layers in order to carry out the electroluminescense operation stably. Therefore, the uniformity and the stability of the insulator layers are some of the most important factors for stabilizing the operations of EL elements and making an EL element highly functional. In addition, the EL element is operated with an AC voltage, the insulator layers work in a manner identical with capacitor films, and accordingly it is necessary to employ insulator layers having high dielectric constants so as to reduce the operating voltage of the EL element.
Moreover, the light-emitting luminance of the EL element is in proportion to the electric current which flows in the luminous layer, and this electric current depends on the charges which accumulate in the boundary surfaces between the insulator layers and the luminous layer. These charges in turn depend upon the maximum accumulable charges of the insulator layers. That is, they depend on the figures of merit of the insulator layers; i.e., the product of the dielectric breakdown fields and the dielectric constants thereof. Hence, it is important to employ a material exhibiting a large maximum accumulable charge in order to make the luminance of the EL element high. Additionally, the EL element does not need to be a propagation type, but it does need to be a self-healing type in the adhesion mode, the dielectric breakdown mode, etc., in the boundary surfaces between the luminous layer and the insulator layers or the boundary surfaces between the insulator layers and the electrodes. If such is the case, even when there arises a dielectric breakdown at a certain portion of the EL element display device, the dielectric breakdown does not propagate over the entire display device, and the other portions thereof which are properly functioning can be in charge of the operation.